Alkenes are hydrocarbons that contain an alkenyl functional group.
They are manufactured industrially by cracking hydrocarbons and are
much more useful than alkanes chemically, as the double bond makes them
reactive towards many other species.

Alkenes are organic compounds that contain a double carbon-carbon (alkenyl)
bond. They are said to be 'unsaturated' in that they can add more hydrogen
atoms to the hydrocarbon skeleton.

The double carbon-carbon bond is a region of high electron density, with
one pair of electrons in a 'pi' orbital, which can be attacked by reagents
containing a positive or partial positive charge. This makes alkenes reactive
and useful as building blocks for organic synthesis.

Alkenes can form addition products with other molecules by 'opening' the
double bond and using the electrons to form bonds at each carbon atom. This
is known as addition.

The process is stimulated by electrophilic attack by the reagent, and for
this reason is called electrophilic addition.

Alkenes react with halogens making disubstituted halogenoalkanes

ethene + bromine
1,2-dibromoethane

C2H4 + Br2
C2H4Br2

Alkenes react with interhalogens making disubstituted halogenoalkanes

ethene + iodine monochloride
1-chloro-2-iodoethane

C2H4 + ICl
CH2ClCH2I

Alkenes react with hydrogen halides making halogenoalkanes:

ethene + hydrogen bromide
bromoethane

C2H4 + HBr
C2H5Br

Alkenes react with steam in the presence of a catalyst making alcohols:

ethene + steam
ethanol

C2H4 + H2O
C2H5OH

Alkenes react with hydrogen in the presence of a Ni catalyst at 150ºC
making alkanes:

ethene + hydrogen
ethane

C2H4 + H2
C2H6

Although it may seem pointless making an unreactive alkane from its useful
alkane, this reaction is important in synthesis, as it allows control of the
degree of unsaturation of long chain compounds that have several double bonds,
such as the vegetable oils.

Changing double bonds to single bonds, via hydrogenation, increases the melting
point of a vegetable oil, an essential step in the production of margarine.

An electrophile is an electron-deficient species that can accept electron
pairs from a nucleophile. Electrophiles are Lewis acids. Deduction of the
mechanism of the electrophilic addition reactions of alkenes with halogens/interhalogens
and hydrogen halides.

Step 1: Attack by the electrophile - the pair of electrons in the pi orbital
of the alkenyl group moves to attach the partially positive side of the
polarised electrophile forming a carbo-cation (positive) intermediate and
breaking the bond in the electrophile heterolytically.

Step 2: The negative ion remaining from the heterolytic fission of the
electrophile then attacks the carbon atom holding the positive charge on
the intermediate.

Alkenes can polymerise in the presence of suitable catalysts. Originally
this was carried out using high pressure and an oxygen catalyst, but nowadays
there are more efficient catalysts containing titanium compounds that can
make better polymeric products (Zeigler-Natta catalysts).

C2H4
-[C2H4]n-

ethene
poly(ethene)

By changing the substituents around the double bond, a large variety of different
polymers can be produced, each one with different properties.